Not Applicable
Not Applicable
1. Field of the Invention
This invention pertains generally to radiation dosimeters, and more specifically to a device for performing optical analysis of track etch foils used within dosimeters for measuring the extent of cumulative radiation exposure.
2. Description of the Background Art
Dosimeters incorporating track etch foils as a recordation medium are commonly used for assessing cumulative radiation exposure. The most commonly used dosimeter, the thermoluminescent detector (TLD), is used to assess several forms of radiation. In addition to the TLD, the PN-3 track etch dosimeter is used to determine the legal dose-of-record from neutron radiation. The PN-3 dosimeters and standardized dosimeters contain three pieces of CR-39(trademark) clear plastic track etch foils which are 1.5 mm thick by 2 cm wide by 2.4 cm in height. Although sensitive to several types of radiation, these track etch foil dosimeters are more generally used for either radon or neutron dosimetry.
Neutron interaction with the plastic within a dosimeter creates material degradation by the action of recoil protons. After the plastic has been etched in a bath of caustic water (6.00 to 6.25 N NaOH) for a specified length of time (usually several hours), the degradation appears as pits (typically of a conical shape) which are of a diameter from two to thirty microns, and barely visible to the naked eye. As a comparison, the diameter of a human hair is typically about ninety microns. The number of pits on the track etch foil per unit area is directly proportional to the radiation exposure, and when compared with the results from foils exposed to a standard amount of radiation, can be used to calculate the cumulative radiation dose received. Currently, the etched pits are counted manually by visual inspection, or by using a photo-optical inspection station which automatically counts the pits.
Visual inspection, using a microscope of sufficient power, enables the user to count the number of pits within a standard field of view. This method produces acceptable results yet is extremely labor intensivexe2x80x94especially at the high end of exposures (above 2 rem). This method is currently used only as a last resort; yet until the advent of this invention it was the only acceptable method for analyzing the PN-3 foils for high levels of neutron exposure.
Photo-optical inspection for low levels of radiation exposure can be performed on commercial instruments which optically count each pit within the track etch foil after it has been etched. One such instrument, the xe2x80x9cAutoscan 60xe2x80x9d (NE Technology, Ltd., Berkshire, England), uses magnification and side-lit illumination from a strong light source to make the pits appear as bright areas against a dark background. A video camera images the surface pits within a unit area, and imaging software then counts the number of pits within that area. This instrument is capable of running a carousel of track etch foils, and producing a report. As described in the published information for the Autoscan system, the upper limit for the basic reading of exposure is up to 2 rems (2000 mrem, or 20 mSv). With the use of xe2x80x9cextraordinary analysisxe2x80x9d, it is claimed that the instrument can read exposures up to 10 rem (10000 mrem, 100 mSv). In practice, the instrument shows a linear range which approaches 1.5 rem (1500 mrem, 15 mSv). It will be recognized that accuracy decreases rapidly at high exposure levels because the pits begin to overlap one another with increasing frequency and the visual pit counting process is not generally unable to discern these overlapping pits. The overlapping of the pits within these high exposure samples may account for the xe2x80x9cfoldbackxe2x80x9d phenomenon experienced when using the Autoscan system.
The drawbacks inherent in these methods of analyzing radiation exposure levels make it difficult to effectively administer proper safety programs. This is especially true in light of recent government requirements to read accumulated radiation levels up to a five rem exposure.
Accordingly, a need exists for an apparatus and method capable of quickly and accurately measuring accumulated radiation exposure as recorded on standard track etch foil patterns up to at least a five rem level of exposure. The present invention satisfies those needs, as well as others, and overcomes the deficiencies of previous solutions.
The present invention is an optical system that addresses the need for accurately reading dosimeter track etch foils to determine cumulative exposure to radiation. The optical system tests the dosimeter track etch foils by passing a collimated light beam through the sample and measuring the exiting scattered light which is largely a function of the density, and size, of the pits in the surface of the foil. Light scattering was observed and mathematically quantified in 1871 by J.W. Strutt (Lord Rayleigh), and is commonly referred to as Rayleigh scattering. Particles registered by Rayleigh scattering are those which are typically smaller than the wavelength of the scattered light. Equations developed by Gustav Mie describe light scattering for particles of a size comparable to the wavelength of the light. Mie scattering is utilized within the present invention for quantifying voids (pits) within the material of the track etch foils. Additional details of the Mie theory can be found in a variety of physics reference textbooks, including in a reference by C. F. Bohren, entitled xe2x80x9cAbsorption and Scattering of Light by Small Particlesxe2x80x9d, ISBN 0-471-05772-X 1983, chapter 4, page 82, and in a reference entitled xe2x80x9cEncyclopedia of Science and Technologyxe2x80x9d, ISBN 0-07-911504-7,1997 page 70 in a section on xe2x80x9cLight Scattering Photometryxe2x80x9d. Both of these publications are incorporated herein by reference.
The present invention is capable of processing dosimeter track etch foils more rapidly and with greater accuracy than presently available automated commercial instruments, while it extends the range of effective dosimeter analysis to include radiation doses up to five rems (5000 mrem, 50 mSv). The invention determines the relative number and size of etched pits by measuring the amount of light which scatters when a collimated beam is passed through a track etch foil. The light is scattered by the pits on the surface of the track etch foil which are indicative of radiation exposure.
By way of example, and not of limitation, in accordance with the present invention after the track etch foils have been chemically etched by conventional means, they are placed in the path of an intense collimated light source, such as a beam from a helium-neon laser (Hexe2x80x94Ne at a wavelength of 632.8 nm), whereby the light is scattered by the pits in the material. The scattered light is collected by a lens, modified by an unscattered light mask, and focused onto a detector or means of registering the intensity of the scattered light. The unscattered light mask blocks the incident beam as it exits from the sample so that the amount of scattered light may be accurately measured, without contributions from the unscattered light. The amount of light scattering which occurs is proportional to pit density and size at the area of beam impingement on the surface of the track etch foil which allows cumulative exposure to be calculated.
Measurements for an unexposed, but chemically etched foil, provide a reference against which scattered light measurements are compared. Unexposed etched reference foils were found to generate negligible amounts of light scatter; only slightly above that which is generated when the track foil sample is removed from the holder to let the light pass through unhindered. When measuring all light exiting an exposed track etch foil, the background light power was found to typically register approximately five times higher than the scattered light when tested at the highest exposure level of five rems. The variation in light intensity received by the detector as a result of light scattering was, thereby, a minor percentage of the overall light power being detected which complicated the calculations of pit density. Masking out the unscattered light allows all remaining light (scattered light) to be registered as indicative of pit density. The unscattered light mask utilized in the present invention is capable of increasing response linearity such that a light intensity of near xe2x80x9czeroxe2x80x9d will approximate an accumulated radiation level near xe2x80x9czeroxe2x80x9d rems since the meter is restricted to detecting only the light scattering in response to the pits within the chemically etched foils. The invention therefore provides a number of benefits which aid the analysis of track etch foils used in dosimeters.
An object of the invention is to improve safety for workers involved with radioactive materials by simplifying testing for cumulative exposure to radiation sources, so that more frequent testing may be performed.
Another object of the invention is to provide an apparatus and method for the rapid measurement of accumulated radiation as registered on track etch foils which typically are used within dosimeters.
Another object of the invention is to provide a method that yields accumulated radiation measurements that are highly accurate.
Another object of the invention is to provide a method capable of measuring high levels of accumulated radiation up to and including at least five rems.
Another object of the invention is to provide a method capable of taking light scatter angles into account to further enhance accuracy of the measured exposure level.
Another object of the invention is to provide a measurement apparatus that may be constructed from generally available materials and devices.
Further objects and advantages of the invention will be brought out in the following portions of the specification, wherein the detailed description is for the purpose of fully disclosing preferred embodiments of the invention without placing limitations thereon.